feat: InMemoryTaskStore creates a copy of Task by default to make it consistent with database task stores

[bartek-w]

Sharing the Task object instance in InMemoryTaskStore leads to unexpected behaviour (from differences of in-place update Task in AgentExecutor to non-trivial concurrency reporting issues on task state reporting).

Fixes #869

[gemini-code-assist]

Summary of Changes

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This pull request addresses a critical issue in the InMemoryTaskStore where sharing Task object instances could lead to unexpected behavior and concurrency problems. By introducing a CopyingTaskStoreAdapter and integrating it into the InMemoryTaskStore by default, the system now consistently handles Task objects as deep copies, preventing unintended in-place modifications and aligning its behavior with persistent database task stores.

Highlights

• New Copying Task Store Adapter: Introduced CopyingTaskStoreAdapter to enforce deep copying of Task objects when interacting with any TaskStore, preventing shared mutable state issues.
• Refactored InMemoryTaskStore: The InMemoryTaskStore has been refactored to internally use _InMemoryTaskStoreImpl and now wraps it with CopyingTaskStoreAdapter by default, ensuring consistent behavior with database-backed task stores.
• Enhanced Test Coverage: Added new integration and unit tests specifically designed to verify the task copying behavior and prevent unintended shared mutable state issues.

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[github-actions]

🧪 Code Coverage (vs 1.0-dev)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/server/tasks/inmemory_task_store.py	96.77%	97.14%	🟢 +0.37%
src/a2a/server/tasks/copying_task_store.py (new)	—	100.00%	—
Total	91.39%	91.43%	🟢 +0.04%

Generated by coverage-comment.yml

[gemini-code-assist]

Code Review

This pull request introduces a significant improvement to the InMemoryTaskStore by making it behave consistently with database-backed task stores. By default, it now returns copies of Task objects instead of direct references, which prevents subtle bugs related to shared mutable state. This is achieved cleanly by introducing a CopyingTaskStoreAdapter and refactoring InMemoryTaskStore to use it. The changes are well-structured and accompanied by a comprehensive set of unit and integration tests that validate the new behavior. My review includes a couple of minor suggestions to improve code clarity and remove unused code. Overall, this is an excellent change.